Patient positioning systems are used for accurate and reproducible positioning of a patient for radiation therapy, surgery, and other medical procedures. During these procedures, it is common to immobilize a part or parts of the patient's body. Accurate positioning of the body part is also important in initial and subsequent treatments, so that the precisely the same location of the body is exposed to the radiation each time. Therefore, different types of devices have been made to immobilize body parts and to index the body to the treatment table to assure proper and repeatable alignment for radiation therapy.
As radiation therapy becomes more precise with procedures such as 3D conformal Intensity Modulated Radiation Therapy (IMRT), it is critical that all facets of treatment planning are handled with maximum accuracy. Presently, when advanced, highly technical radiation therapy is performed, an imaging scan, such as computed tomography (CT), is obtained with the patient “in treatment position.” From this scan, not only are the fields and blocking designed, but the treatment plan is calculated.
The standard CT scan table has a hard, convex upper patient support surface, as generally indicated in FIG. 1 below. In practice, this table is provided with a flexible mat which conforms to the upper convex surface of the table, the primary purpose of which is to improve patient comfort during extended scanning. While this configuration contributes to patient comfort, the convex structure of the table makes it difficult to exactly index the scanning data obtained to the conditions that will subsequently be encountered during radiation treatment. CT scans have also been performed on a standard CT scan “flat tabletop insert” to enable better positioning of the patient for advanced treatments. The flat tabletop inserts raise the patient above the level of the of the sidewalls of the CT table as shown in FIG. 1C, facilitating scanning of the entire body. This table insert is flat, very hard, allows for negligible deflection (defined as the downward movement of the tabletop) under weight of the patient, and tends to flatten the body parts to be imaged. However, in subsequent radiation oncology procedures, a mesh section of the typical radiation oncology table is usually in place in the simulator/linear accelerator during treatments. When positioning a patient for treatment, a deflection of the mesh occurs under weight of the patient, which can be as much as two to three centimeters at the maximum point.
Lack of accurate position representation at the time of the CT scan will not allow advanced radiation therapy to be as accurate as possible. Higher accuracy can be obtained if the scan is performed directly in accordance with the shape and position of the patient when positioned on the linear accelerator table, especially as the accuracy in IMRT and the like equipment improves and provides greater accuracy and adjustability in treatment and treatment planning.
Current problems with accurate and precise patient positioning for radiation therapy include deflection in treatment/simulator table that is different from deflection in a table used for imaging, variations in patient location on the linear accelerator table varying from day to day, lack of alignment (leveling) of table inserts used in imaging, and the fact that many facilities have already purchased and scan using table inserts that do not possess deflection capabilities. Every CT, simulator, and linear accelerator deflection should match if the patient is to be treated accurately and reproducibly.